As an exhaust gas purification system that purifies and exhausts exhaust gas of a diesel engine, an exhaust gas purification system in which a DPF (Diesel Particulate Filter) and an SCR (Selective Catalytic Reduction) device are connected to an exhaust pipe has been developed.
The exhaust gas purification system uses the DPF to trap a PM in exhaust gas and uses an SCR system including the SCR device to supply urea water stored in a urea tank to an exhaust gas upstream of SCR, generate NH3 by the heat of the exhaust gas, and reduce and purify NOx on an SCR catalyst by the NH3 (see, for example, Patent Document 1).
Since the PM trapped by the DPF causes filter clogging, it is necessary to appropriately oxidate and remove the trapped and deposited PM for regeneration.
When an exhaust gas pressure sensor detects a differential pressure before and after the DPF and the differential pressure reaches an upper limit value, an ECM (Engine Control Module) automatically or manually detects the filter clogging. In this case, a DPF warning lamp provided in a cabin is lighted and a driver presses a regeneration execution switch, thereby starting DPF regeneration.
The DPF regeneration is performed by raising a temperature of the exhaust gas to about 600° C. and burning and removing the PM trapped by the DPF by the high-temperature exhaust gas. That is, in the DPF regeneration, since the high-temperature exhaust gas is flowed into the exhaust pipe, the DPF and the SCR device connected to the exhaust pipe are exposed to the high-temperature exhaust gas.
By the way, due to the nature of the SCR catalyst, the amount of NH3 capable of being accumulated by the SCR catalyst decreases as the temperature rises. Therefore, when the DPF regeneration is started with a large amount of NH3 accumulated in the SCR catalyst, the SCR catalyst is heated by the high-temperature exhaust gas, so that a so-called “NH3 slip” phenomenon in which the NH3 accumulated in the SCR catalyst is released occurs.
In order to avoid the NH3 slip, the DPF regeneration is performed after reducing the amount of NH3 accumulated in the SCR catalyst by stopping the supply of the urea water prior to the DPF regeneration and purifying the exhaust gas solely by the NH3 accumulated in the SCR catalyst.